1. Field of the Invention
This invention relates generally to the mooring of tankers or other vessels in unprotected waters. More particularly, the invention relates to a mooring system which combines a submerged buoyant mooring element anchored to the seabed by catenary lines and a vessel provided with a hoisting apparatus for raising the mooring element to the vessel and apparatus for securing the vessel to the mooring element independently of the hoisting apparatus.
2. Background Art
In recent years, a number of undersea oil and gas fields have been developed in offshore areas that are subject to extreme weather conditions. Oil or gas is typically delivered from a wellhead on the sea floor to a semi-permanently moored converted tanker or to a special purpose vessel known as a floating storage and off-loading (FSO) vessel or a floating production storage and off-loading (FPSO) vessel. These vessels are designed to remain on station permanently, unless oncoming severe storms or ice floes threaten damage to or loss of the vessel. In such an event, the well is shut in and the vessel is unmoored and sails or is towed away. Upon passing of the storm or ice floe condition, the vessel returns and is again moored above the wellhead.
A typical mooring system, such as is described, for example, in U.S. Pat. Nos. 4,604,961 and 4,892,495, includes a buoyant mooring element that is connected to the wellhead by a flexible pipe and to the sea floor by a number of angularly spaced catenary lines. The system is arranged so that the weight of the portions of the catenary lines that are not resting on the sea bottom counteracts the buoyancy of the mooring element to maintain it normally at a predetermined submerged depth, called the stowed position, when no vessel is moored to it. This assures that the mooring element will not be a navigation hazard or be damaged by a collision.
Mooring is accomplished by pulling the submerged mooring element up to the vessel and securing it rigidly by mechanical latching or clamping devices to a rotary turret mounted in a recess or well in the bottom of the vessel. The securing apparatus must withstand separating forces acting between the vessel and the mooring element during higher sea states than the maximum sea state that will permit the mooring operation. The connecting elements of the securing apparatus thus need to be large and heavy and to be precision made so as to mate perfectly for avoiding stress concentrations. Not only does the requirement for precision increase the time to fabricate the securing apparatus and its cost, but also the relatively close fit between the components on the vessel and those on the mooring element demands careful alignment of the mating surfaces during mooring. Thus, this operation can take place only in a relatively low sea state, and the time required to wait for the sea state to subside after the storm has passed is additional lost production time from the field.
Another problem with the present technology is the possibility of damage to the vessel or the mooring element during the mooring operation because a relatively long time is required to secure the mooring element to the turret after the hoist line has brought the mooring element close to the vessel bottom. During this time a number of rolling and pitching cycles of the vessel may take place in even moderate sea states, and these movements can result in multiple collisions between the vessel and the mooring element, causing bending and denting of the mating surfaces.
Finally, unmooring usually is accomplished by releasing the latching or clamping connectors while they are under load. This procedure also has an inherent risk of structural deformation and damage to the connectors, particularly if there is any binding or delay in releasing some of the connectors during disconnect. Repairs to damaged mechanical mooring components can mean additional weeks of costly oil field production shutdown.